1. Field of the Invention
The present invention relates to an inclination angle detection apparatus and an automobile theft alarm apparatus using the same based on detecting the variation of the inclination angle of the automobile.
2. Description of the Prior Art
The stealing of tires from a parked automobile by jacking up the automobile or a theft of the automobile itself by using a wrecker can be sensed by monitoring the variation of the angle of inclination of the automobile. The inclination angle itself of the automobile can be easily sensed using an acceleration (G) sensor or other inclination sensing means, but in the case of detecting the variation of the inclination angle, the inclination angle when the automobile was parked must be referenced.
An inclination variation detecting means using direct currents stores the initial inclination angle of the automobile when the automobile was parked so that the variation of the inclination angle at any particular point of time is obtained by comparing the initial angle with the inclination angle at that particular point of time. Therefore, a memory for storing the initial inclination angle is required.
On the other hand, an inclination variation detecting means using alternating-currents obtains the variation of the inclination angle by differentiating the output of an inclination sensing means, and therefore, the above-said memory is not required.
An example of an apparatus for detecting the variation of the inclination angle of the automobile using alternating-currents is disclosed in Japanese Patent Publication No. 63-255664. The apparatus is shown in FIGS. 1(a)-1(b), wherein the numeral 111 designates an acceleration sensor mounted in the automobile, element 112 is a constant current circuit, and element 113 is a differential amplifier, which combine to constitute an inclination sensing means 110. The element 120 is an HPF (high-pass-filter) for obtaining only the amount of variation of the inclination angle from the output of the inclination sensing means 110, and element 131 is an AC amplifier for amplifying the output of the HPF 120.
The differential amplifier 113 for amplifying an output of the acceleration sensor 111 consists of two stages of operational amplifiers a1 and a2. The HPF 120 is used to extract only the alternating-current component (the amount of variation of the inclination angle) from the output of the inclination sensing means 110. In theory, it may consists only of a coupling capacitor ca, but in this example, a resistor ra is inserted to provide a high-pass characteristic. The AC amplifier for amplifying the output of the HPF 120 consists of an operational amplifier a3.
When the constant current I is 5 mA, the sensitivity of the acceleration sensor 111 is approximately 1.35 mV/0.5 G. G is the acceleration of free fall, and 0.5 G is equivalent to the inclination of 30 degrees. The qualification of resolution required for sensing the theft of the automobile is as small as 3 degree in terms of the detection angle, which is about 0.0523 G in the acceleration of free fall. Therefore, supposing the threshold value of the rear stage circuit is 0.5 V, the required total gain of the amplifiers 113 and 131 is: EQU 0.5(V)/{[1.35(mV).times.0.0523(G)]/0.5(G)}=3540(times)
Also, in order to sense a slow jacking-up action, e.g., an inclination of 3 degrees over 30 seconds, the HPF 120 is required to have a cut-off frequency of 0.0016 Hz, which means very large time constants of ca and ra.
Large time constants of ca and ra, as mentioned above, will present difficulty in the selection of the capacitor ca. That is, using a large-capacitance film capacitor as the capacitor ca will result in an increase not only in size but also in cost. On the other hand, using an electrolytic capacitor as the capacitor ca will involve the problem of causing an offset because of leakage current. For example, even if a leakage current as small as 0.6 .mu.A flows in the 1M.OMEGA. resistor ra, it will generate an offset voltage of 0.6 V, which is enough to erroneously activate an alarm means having a threshold value of 0.4 to 0.5 V.